In many applications, such as spectroscopy and spectral interferometry, there is a need for a wavelength tunable light source with broad optical bandwidth and rapid tuning speed. In many such applications the spectral purity of the light source is important in order to avoid parasitic signals, e.g. seen as a decrease in dynamic coherence length.
Wavelength tunable laser sources, such as tunable vertical cavity surface emitting lasers (VCSELs), are attractive for a number of applications, such as spectroscopy and optical coherence tomography (OCT). Micro-electro-mechanical system (MEMS) tunable vertical cavity surface emitting diodes (VCSELs) have the potential of enabling truly single-mode wavelength tuning with more than 10% relative tuning range and/or single-mode wavelength tuning at up to 100 MHz tuning rates. The compactness of VCSEL sources is another attractive feature for many industrial applications. Compact swept sources made by fabrication of electrical-injection VCSELs further allow wafer-scale testing.
However, in order to achieve some of the above properties of MEMS tunable VCSELs a number of issues remain to be addressed. Firstly the electro-mechanical instability of the air gap parallel-plate capacitor of a tunable VCSEL limits the achievable optical bandwidth. Operation of a tunable VCSEL laser involves the risk that the tunable reflector collapses onto the underlying substrate, a so-called “pull-in” or “snap-down”, involving the risk of permanent damage of the laser source. Consequently, prior art laser sources typically only utilise a fraction of the maximum tuning amplitude of tunable reflector. In particular, the parallel-plate electro-mechanical actuator has been found to be limited by pull-in phenomena where, as the electrostatic force increases beyond the mechanical restoring force, the parallel-plate snaps to the substrate (in this case the VCSEL substrate).
Secondly, a wide deflection of the movable part of the parallel-plate actuator is desirable.
From Cole et al., Optics Express, vol. 16, (2008), p. 16093 is known a short-wavelength MEMS-tunable VCSEL, comprising DBR top and bottom reflectors, and an anti-reflection coating within the cavity. A wavelength tuning range of 30 nm is reported.
From Vail et al. Electronics Letters 32 (1996) 1888 and Jayaraman et al. Electronics Letters 48 (2012) are known other laser sources capable of tuning.
From Vail et al. IEEE Journal of Selected Topics in Quantum Electronics, vol. 3 (1997) pp. 691 is known that an alternating current (AC) voltage can be used to oscillate the MEMS oscillator at either side of the rest position thus both providing blue-shift and red-shift of the wavelength. The air gap is designed to be 1.41 μm, or 3/2λ in air, which results in a MEMS-oscillator safe stroke length of 470 nm (that is the stroke length than can be accommodated before pull-in will occur). Vail et al. describe that the voltage required for a given wavelength change can be reduced by driving the MEMS oscillator at resonance with a square waveform with peak voltage of 16 V. In this way the VCSEL can be swept across its full tuning range of 12 nm. With a wavelength tuning efficiency of 0.04 the required stroke of the MEMS oscillator to achieve the full tuning range is 300 nm. Given that the MEMS oscillator deflects at either side of the rest position the required 150 nm downward deflection is within the stable region and the pull-in instability is avoided. Larger deflections of the MEMS oscillator using a square waveform would result in dynamic pull-in. The dynamic pull-in instability is known from e.g. Seeger et al. Solid-State Sensor, Actuator and Microsystems Workshop Jun. 2-6 (2002) 0-9640024-4-2, which teaches that pull-in occurs at 56% of the air gap in the rest position for square-wave excitation at the resonance of the MEMS oscillator.
Jayaraman et al. teach in Proc. SPIE vol. 8276 (2012) pp. 82760D, Electronics Letters vol. 48 (2012) pp. 867-869 how the ⅓ gap rule can be exceeded under repetitive sinusoidal sweeping. The static snap-down instability can be exceeded by repetitive sweeping, but the dynamic snap-down instability will still limit the tuning range.
Hence, an improved laser source and laser source system would be advantageous, and in particular a laser source and laser source system having an extended tuning range would be advantageous.
It is further desirable to reduce the risk of damaging the laser source during operation.
In particular, it may be seen as an object of the present invention to provide a laser source, a laser source system and a method of use of a laser source that increases the tuning range of laser sources of the prior art.
It is a further object of the present invention to provide an alternative to the prior art.